Kinetics of Bromine−Magnesium Exchange Reactions in Substituted Bromobenzenes

Competition experiments have been performed to determine the relative reactivities of substituted bromobenzenes, bromonaphthalenes, and 9-bromoanthracene toward i-PrMgCl·LiCl in THF at 0 °C. The rates of the bromine−magnesium exchange reactions are accelerated by electron-acceptor substituents, the activating efficiency of which increases in the order para < meta ≪ ortho. The activation free enthalpies of the bromine−magnesium exchange reactions correlate fairly (r 2 = 0.83) with the proton affinities of analogously substituted aryllithiums (slope 0.8). The kinetics of two representative bromoarenes with i-PrMgCl·LiCl were found to be first-order in both bromoarene and i-PrMgCl·LiCl. Combination of the resulting second-order rate constants with the k rel values from competition experiments allowed us to calculate reaction times for the bromine−magnesium exchange reactions of a large variety of bromoarenes.